This invention relates to the detection of the occurrence of an apnea (i.e. the complete cessation of breathing) and to the determination of airway patency. The condition of patency of the airway is the converse of a total obstruction of the airway. The invention also relates to the detection of partial obstruction of the airway (i.e. obstructed breathing). The detection and monitoring of apneas, airway patency and obstruction is advantageous in the diagnosis and treatment of respiratory conditions that have adverse effects on a person""s wellbeing.
The expression xe2x80x9cairwayxe2x80x9d as used herein is to be understood as the anatomical portion of the respiratory system between the nares and the bronchii, including the trachea. The expression xe2x80x9crespirationxe2x80x9d is to be understood as the continually repeating events of inspiration (inhaling) followed by expiration (exhaling).
In the Sleep Apnea syndrome a person stops breathing during sleep. Cessation of airflow for more than 10 seconds is called an xe2x80x9capneaxe2x80x9d. Apneas lead to decreased blood oxygenation and thus to disruption of sleep. Apneas are traditionally (but confusingly) categorized as either central, where there is no respiratory effort, or obstructive, where there is respiratory effort. With some central apneas, the airway is patent, and the subject is merely not attempting to breathe. Conversely, with other central apneas and all obstructive apneas, the airway is not patent (i.e. occluded). The occlusion is usually at the level of the tongue or soft palate.
The airway may also be partially obstructed (i.e. narrowed or partially patent). This also leads to decreased ventilation (hypopnea), decreased blood oxygenation and disturbed sleep.
The dangers of obstructed breathing during sleep are well known in relation to the Obstructive Sleep Apnea (OSA) syndrome. Apnea, hypopnea and heavy snoring are recognised as causes of sleep disruption and risk factors in certain types of heart disease. More recently it has been found that increased upper airway resistance (Upper Airway Resistance syndrome) during sleep without snoring or sleep apnea also can cause sleep fragmentation and daytime sleepiness. It is possible there is an evolution from upper airway resistance syndrome to sleep apnea, accompanied by a worsening of clinical symptoms and damage to the cardiovascular system.
The common form of treatment of these syndromes is the administering of Continuous Positive Airway Pressure (CPAP). The procedure for administering CPAP treatment has been well documented in both the technical and patent literature. Briefly stated, CPAP treatment acts as a pneumatic splint of the airway by the provision of a positive pressure, usually in the range 4-20 cm H2O. The air is supplied to the airway by a motor driven blower whose outlet passes via an air delivery hose to a nose (or nose and/or mouth) mask sealingly engaged to a patient""s face. An exhaust port is provided in the delivery tube proximate to the mask. More sophisticated forms of CPAP, such as bi-level CPAP and autosetting CPAP, are described in U.S. Pat. Nos. 5,148,802 and 5,245,995 respectively.
Various techniques are known for sensing and detecting abnormal breathing patterns indicative of obstructed breathing. U.S. Pat. No. 5,245,995, for example, describes how snoring and abnormal breathing patterns can be detected by inspiration and expiration pressure measurements while sleeping, thereby leading to early indication of preobstructive episodes or other forms of breathing disorder. Particularly, patterns of respiratory parameters are monitored, and CPAP pressure is raised on the detection of pre-defined patterns to provide increased airway pressure to, ideally, subvert the occurrence of the obstructive episodes and the other forms of breathing disorder.
As noted above, central apneas need not involve an obstruction of the airway, and often occur during very light sleep and also in patients with various cardiac, cerebrovascular and endocrine conditions unrelated to the state of the upper airway. In those cases where the apnea is occurring without obstruction of the airway, there is little benefit in treating the condition by techniques such as CPAP. Also, known automated CPAP systems cannot distinguish central apneas with an open airway from apneas with a closed airway, and may inappropriately seek to increase the CPAP splinting air pressure unnecessarily. Such unnecessary increases in pressure reflexly inhibit breathing, further aggravating the breathing disorder.
Other limitations associated with the prior art include the inability to detect airway patency and the absence of progressive, heirarchic response to increasingly severe indicators of airway obstruction for which the mask pressure should be increased.
It would be useful, however, to even more sensitively and reliably detect the conditions of partial obstruction, as well as apnea and patency, as this would assist in the design of equipment to prevent these conditions from occurring. In a similar way, means for detecting and monitoring mildly obstructed breathing would be useful in diagnosing and treating Upper Airway Resistance syndrome and monitoring that treatment is optimal.
In accordance with a first aspect the invention discloses a method for determining the occurrence of an apnea in a patient, the method comprising the steps of:
measuring respiratory air flow from the patient as a function of time;
determining the variance of said measured air flow; and
determining from said variance that an apnea is occurring.
The variance can be a moving average over the time window. Further, there can be a further step of comparing the variance with a threshold value, and if the variance falls below the threshold value then an apnea is occurring. The measured air flow can be expressed as an air flow signal. Advantageously, the respiratory air flow is sampled at equally spaced points in time to give a sampled air flow signal. Further, it can be the case that the variance must fall below the threshold value for a predetermined period of time before it is determined that an apnea is occurring. Advantageously the method can comprise the further step of either commencing continuous positive airway pressure (CPAP) treatment or increasing CPAP treatment pressure to the patient if an apnea is occurring.
In accordance with a further aspect the invention discloses a method for detecting partial obstruction of the airway of a patient, the method comprising the steps of:
measuring respiratory air flow from the patient;
detecting the inspiratory part of said air flow;
normalising said inspiratory part; and
determining an index value of a mid-portion of said normalised inspiratory part as a measure of partial obstruction.
Conveniently, the index value is determined from the amplitude of the mid-portion of the normalised inspiratory part. The index value can be determined as the arithmetic mean value of the amplitude in the mid-portion. Alternatively, the index value is determined from the flatness of the mid-portion. Yet further, the index value can be determined as the root mean square (RMS) deviation of the normalised inspiratory part in the mid-portion with respect to unity. The RMS deviation can be compared against a threshold value to determine the degree of obstruction. Still further, the step of normalising can include scaling the inspiratory part to unity duration and unity area. The determination also can be performed over a plurality of inspiratory events. In this way a moving mean value of amplitude or a moving RMS deviation can be formed. The respiratory air flow also can be sampled at spaced instants in time. Advantageously there is the further step of either commencing CPAP treatment or increasing CPAP treatment pressure to the patient if there is partial obstruction of the airway.
The invention yet further discloses a method for determining the degree of obstruction of the airway of a patient receiving continuous positive airway pressure (CPAP) treatment by apparatus for supplying CPAP to the patient""s airway, the method comprising the steps of:
measuring respiratory air flow from the patient to give an air flow signal;
filtering said air flow signal to reject components at least due to respiration to give a filtered air flow signal having components due to patient snoring and noise of said CPAP apparatus;
predicting a CPAP apparatus noise component of said filtered air flow signal; and
subtracting said predicted noise component from said filtered air flow signal to give a snore component signal as a measure of the degree of obstruction of the airway.
In a preferred form, the filtering step includes bandpass filtering also to reject high frequency noise components.
The invention yet further discloses a method for determining patency of the airway of a patient, the method comprising the steps of:
applying an oscillatory pressure waveform of known frequency to the patient""s airway;
measuring respiratory air flow from the patient; and
determining that the airway is patent if there is a component of said air flow at said known frequency induced by said oscillatory pressure waveform.
Advantageously the air flow component is determined from the amplitude of the air flow signal, and there is the further step of comparing the magnitude with a threshold value and if the magnitude is greater than the threshold value then the airway is declared patent. Furthermore, the method can be performed when the patient is having an apnea and there is zero air flow. The step of determining can be said to identify modulation of the measured air flow by the oscillatory pressure waveform.
The invention yet further discloses a method for determining the degree of patency of the airway of a patient, the method comprising the steps of:
applying an oscillatory pressure waveform of known frequency and magnitude at an entrance to the patient""s airway;
measuring respiratory air flow from the patient;
determining the magnitude of the component of said air flow at said known frequency induced by said oscillatory pressure waveform; and
determining the degree of patency as the ratio of said induced air flow magnitude and said oscillatory pressure waveform magnitude.
The measured air flow can be expressed as an air flow signal. Furthermore, the method can be performed when the patient is asleep, and further advantageously, when it previously has been determined that the patient is having an apnea. In the case of an apnea there is zero air flow.
The invention yet further discloses a method for determining patency of the airway of a patient, the method comprising the steps of:
measuring respiratory air flow from the patient;
analysing said air flow to detect the presence of cardiogenic air flow, and if said cardiogenic air flow is present then the airway is declared patent.
Again, the measured air flow can be expressed as an air flow signal. The respiratory air flow can be high pass filtered to reject components due to respiration. Further, the step of analysing detects the presence of a periodic component. The periodic component can include a fundamental component together with a sub-multiple or harmonic thereof. The step of analysing can further include performing a Fourier transformation on the air flow signal. Conveniently there can be the further step of detecting the patient""s cardiac rate, and whereby the analysing step includes detecting a component of the air flow at the cardiac rate. Furthermore, the method can be performed when the patient is having an apnea and there is zero air flow.
The invention yet further discloses a method for controlling the administration of CPAP treatment to the airway of a patient by means controllable to supply breathable air to the patient""s airway continually at a selectable pressure elevated above atmospheric pressure, the method comprising the step of:
commencing or increasing CPAP treatment pressure if;
(a) an apnea is occurring, determined by the steps of;
measuring respiratory air flow from the patient as a function of time; and
determining the variance of said measured air flow as an indication of an apnea occurring;
or (b) there is partial obstruction of the airway, determined by the steps of:
measuring respiratory air flow from the patient;
detecting the inspiratory part of said air flow;
normalising said inspiratory part; and
determining an index value of a mid-portion of said normalised inspiratory part as a measure of partial obstruction;
or (c) there is patency of the airway, determined by the steps of:
(i) applying an oscillatory pressure waveform of known frequency to the patient""s airway;
measuring respiratory air flow from the patient; and
determining that the airway is patent if there is a component of said air flow at said known frequency induced by said oscillatory pressure waveform;
or (ii) measuring respiratory air flow from the patient; and
analysing said measured air flow to detect the presence of cardiogenic air flow, and if so then the airway is declared patent.
The invention yet further discloses apparatus for determining the occurrence of an apnea in a patient, the apparatus comprising:
means for measuring respiratory air flow from the patient as a function of time;
means for determining the variance of said measured air flow; and
means for determining from said variance that an apnea is occurring.
The invention yet further discloses apparatus for detecting partial obstruction of the airway of a patient, the apparatus comprising:
means for measuring respiratory air flow from the patient;
means for detecting the inspiratory part of said air flow;
means for normalising said inspiratory part; and
means for determining an index value of a mid-portion of said normalised inspiratory part as a measure of partial obstruction.
The invention yet further discloses apparatus for determining the degree of obstruction of the airway of a patient receiving continuous positive airway pressure (CPAP) treatment by means for supplying CPAP to the patient""s airway, the apparatus comprising:
means for measuring respiratory air flow from the patient to give an air flow signal;
means for filtering said air flow signal to reject components at least due to respiration to give a filtered air flow signal having components due to patient snoring and noise of said CPAP apparatus;
means for predicting a CPAP apparatus noise component of said filtered air flow signal; and
means for subtracting said predicted noise component from said filtered air flow signal to give a snore component signal as a measure of the degree of obstruction of the airway.
The invention yet further discloses apparatus for determining patency of the airway of a patient, the apparatus comprising:
means for applying an oscillatory pressure waveform of known frequency to the patient""s airway;
means for measuring respiratory air flow from the patient; and
means for determining that the airway is patent if there is a component of said air flow at said known frequency induced by said oscillatory pressure waveform.
The invention yet further discloses apparatus for determining the degree of patency of the airway of a patient, the apparatus comprising:
means for applying an oscillatory pressure waveform of known frequency and magnitude to the patient""s airway;
means for measuring respiratory air flow from the patient;
means for determining the magnitude of the component of said air flow at said known frequency induced by said oscillatory pressure waveform; and
means for determining the degree of patency as the ratio of said induced air flow magnitude and said oscillatory pressure waveform magnitude.
The invention yet further discloses apparatus for determining patency of the airway of a patient, the apparatus comprising:
means for measuring respiratory air flow from the patient; and
means for analysing said measured air flow to detect the presence of cardiogenic air flow, and if said cardiogenic air flow is present then the airway is declared patent.
The invention yet further discloses apparatus for controlling the administration of CPAP treatment to the airway of a patient comprising
means controllable to supply breathable air to the patient""s airway continually at a selectable pressure elevated above atmospheric pressure;
controlling means for commencing or increasing CPAP treatment pressure if:
(a) an apnea is occurring, determined by:
measuring respiratory air flow from the patient as a function of time; and
determining the variance of said measured air flow as an indication of an apnea occurring;
or (b) there is partial obstruction of the airway, determined by:
measuring respiratory air flow from the patient;
detecting the inspiratory part of said air flow;
normalising said inspiratory part; and
determining an index value of a mid-portion of said normalised inspiratory part as a measure of partial obstruction;
or (c) there is patency of the airway, determined by:
(i) applying an oscillatory pressure waveform of known frequency to the patient""s airway;
measuring respiratory air flow from the patient; and
determining that the airway is patent if there is a component of said air flow at said known frequency induced by said oscillatory pressure waveform;
or (ii) measuring respiratory air flow from the patient; and
analysing said measured air flow to detect the presence of cardiogenic air flow, and if so then the airway is declared patent.